Many studies have conventionally been made to reduce fuel consumption by reducing rolling resistance (expressed also as “by improving rolling resistance performance”) of tires. Further reduction in fuel consumption has been more and more strongly required in recent years, and a rubber composition for tire components, such as a tire tread, is required to have more excellent low-exothermic property.
Known as a method for satisfying low-exothermic property of a rubber composition is a method in which an amount of a reinforcing filler is reduced. In such a case, however, hardness of the rubber composition decreases. On the other hand, a rubber composition desirably has a rubber hardness (Hs) of about 55 in order to maintain handling stability. Thus, it is important for the rubber composition containing the reinforcing filler in a decreased amount to inhibit decrease in Hs and to ensure proper breaking strength. The breaking strength, particularly elongation at break [EB (%)], relates to cut resistance and separation growth resistance. It is also important to improve crack growth resistance of rubber in order to prevent cracks on a groove of a tread from growing to cause burst owing to separating off.
For ensuring proper breaking strength of the rubber composition containing a reinforcing filler in a decreased amount, a method in which silica is substituted for carbon black, and a method in which a predetermined amount of natural rubber is blended may be contemplated. However, polymers containing natural rubber blended with butadiene rubber or the like easily cause reversion, which causes decrease in Hs and breaking strength. Even in the case of substitution by silica, polymers and silica are not strongly bound to one another, which causes insufficient reduction of heat generation. For example, in the case of using a BR with a tin-modified end as butadiene rubber, the BR with a tin-modified end is strongly bound to CO and COOH on the surface of carbon black, but it is not strongly bound to silica; thus, it is difficult to reduce heat generation. Furthermore, in the case of reducing a filler content, its dispersibility worsens rather than improves, which causes decrease in breaking strength and increase in heat generation.
Patent Document 1 discloses that a styrene-butadiene rubber that is modified by a predetermined organosilicon compound containing an alkoxy group improves rolling resistance and grip performance. However, there is still a room for improvement in order to satisfy all of low-exothermic property, breaking strength, and crack growth resistance of rubber. Furthermore, the document discloses, for the most part, a rubber with a styrene content of 20 to 30%, and there is no study on an optimal styrene content for each of compositions for a base tread, for coating a carcass, and for a tie gum of a tire.
Patent Document 1: JP 2001-114938 A